Mahler, L. 2005. Record of Abelisauridae (Dinosauria: Theropodia)

Journal of Vertebrate Paleontology 25(1):236–239, March 2005
© 2005 by the Society of Vertebrate Paleontology
NOTE
RECORD OF ABELISAURIDAE (DINOSAURIA: THEROPODA) FROM THE CENOMANIAN OF MOROCCO
LUKE MAHLER*, Department of Anatomy and Organismal Biology, University of Chicago, 1027 East 57th Street, Chicago, Illinois 60637
U.S.A.
Abelisauroid theropods have1 been used as evidence of land connections between several southern continents and Europe during the Late
Cretaceous. Abelisauroids (abelisaurids and noasaurids) have been described from South America (Bonaparte and Powell, 1980; Bonaparte
and Novas, 1985; Martínez et al., 1986; Bonaparte et al., 1990; Coria et
al., 2002; Lamanna et al., 2002), Madagascar (Krause and Hartman, 1996;
Sampson et al., 1996, 1998, 2000, 2001; Carrano et al., 2002), India (Chatterjee, 1978; Wilson et al., 2003), and southern Europe (Buffetaut et al.,
1988; Le Loeuff and Buffetaut, 1991; Accarie et al., 1995). Their presence
in Africa has been in dispute.
Russell (1996) referred dentary fragments from the Kem Kem beds of
Morocco (Cenomanian; Sereno et al., 1996) to the Abelisauridae based
on small size and the subrectangular shape of the alveoli. This assignment, however, has been questioned (Sampson et al., 1998; Carrano et
al., 2002). Smith et al. (2001) have speculated that various isolated teeth
from the Bahariya Formation (Cenomanian) of Egypt are abelisaurid,
but this also remains a tentative assignment. Recently, Sereno et al.
(2002, 2004) have described a noasaurid and an abelisaurid from the
Aptian-Albian of Niger, and have reinterpreted Deltadromeus agilis, of
the Cenomanian of Morocco (upper Kem Kem), as a possible basal
noasaurid. A new maxilla, described below, establishes without doubt
the presence of abelisaurids on the northwestern shores of Africa during
the Cenomanian.
Institutional Abbreviations—FMNH, Field Museum of Natural History, Chicago; UCPC, University of Chicago Paleontological Collection,
Chicago; UNPSJB, Universidad Nacional de Patagonia “San Juan
Bosco,” Comodoro Rivadavia, Argentina.
SYSTEMATIC PALEONTOLOGY
THEROPODA Marsh, 1881
ABELISAURIA Novas, 1992
ABELISAUROIDEA Bonaparte, 1991
ABELISAURIDAE Bonaparte and Novas, 1985
gen. et sp. indet.
Referred Specimen—UCPC 10, a partial right theropod maxilla.
Locality and Stratigraphic Horizon—Specimen UCPC 10 was collected by locals in the Kem Kem beds, probably near Erfoud in western
Morocco. The Kem Kem beds record alternating delta and nearshore
marine environments and consist of channel deposits of red sandstone
(Russell, 1996; Sereno et al., 1996) identical to the matrix that encrusted
the specimen.
Geological Age—Cenomanian, based on teeth recording the presence
of nine elasmobranchs in this formation, five of which are known exclusively from the Cenomanian (Sereno et al., 1996).
Description—The specimen, consisting of the anterior half of the right
maxilla, is missing posterior and ascending rami, as well as the anteromedial process (Figs. 1, 2). The anteromedially beveled articular surface
for the premaxilla is present. The preserved portion of the ventral margin
is complete, and includes the first six alveoli, all with erupting teeth.
Dorsally, the surface is nearly entirely broken, revealing the interior of
the promaxillary fossa (promaxillary recess of Whitmer, 1997), an en-
* Present address: Department of Biology, Campus Box 1137, Washington University, St. Louis, Missouri 63130, [email protected]
closed pneumatic cavity that extends anteriorly from the anteroventral
corner of the antorbital fenestra. The broken posterior margin has exposed an alveolus in cross-section, which occupies more than half of the
depth of the ramus. Both the lamina medialis and lamina lateralis are
preserved in the posterodorsal corner of this specimen. As is typical of
abelisaurids, these laminae are of nearly equal height and show no external infolding or depression (Figs. 1A, 2A). In non-abelisaurid theropods, the lamina medialis is generally significantly taller than the lamina
lateralis, and a marked depression, the antorbital fossa, is present on the
dorsal margin of the lamina lateralis. In UCPC 10, the smooth dorsal
edge of the lamina lateralis indicates that the maxilla lacked a maxillary
fenestra. The maxilla is very dorsoventrally deep, as its vertical height at
the anterior corner of the antorbital fenestra is approximately 75% of the
horizontal length of the first six alveoli.
The lateral surface of the maxilla is rugose, with large numbers of pits
as well as adjacent grooves and broader channels with rounded edges
(Figs. 1A, 2A). These grooves and channels are not uniform in direction
but generally trend dorsoventrally. The pits open into foramina that pass
into the body of the maxilla. Despite the extensive pitting, there is no
canal along the floor of the antorbital rim for vascular supply. The sutural
contact surface with the premaxilla is near vertical in orientation, although laterally, a flange of the lateral lamina extends forward to overlap
the premaxilla. The ventral edge of the maxilla is gently convex under the
first and second alveoli. The position of each alveolus is marked by a
slight embayment in the lateral wall of the alveolus. Distinctive vascular
pits are present on the ventral margin between each pair of alveoli.
Medially, the thicker dorsal portion of the maxilla overhangs the thinner alveolar housing, creating a horizontal palatal shelf (Figs. 1B, 2B).
The medial surface below and above the palatal shelf is marked by faint,
shallow striations. Below the shelf, the laminar surface is characterized
by a series of medial convexities that overlie the crypts for replacement
teeth. They are more pronounced anteriorly. Although this surface exhibits convexities, there is no separation of interdental plates; they are
fused seamlessly. Because the medial lamina does not extend as far
ventrally as the lateral lamina, more of the tooth sockets are visible in
medial aspect. As with the lateral lamina, the medial lamina is gently
ventrally concave over each alveolus. The palatal shelf itself is largely
broken, and has been reconstructed. As with the lower surface, this shelf
shows an undulating alternation of convexities and concavities that correspond with tooth replacement series. The palatal shelf is higher anteriorly, sloping ventrally as it extends posteriorly. In the anterodorsal
corner of this specimen, the base of the anteromedial process is present.
The edge of a shallow depression or fossa is apparent immediately posterior to this process.
In anterior view, the maxilla is slightly laterally convex just above the
tooth row (Fig. 1D). The remainder of the body of the maxilla is flat.
Several small foramina are visible on the anterior surface of the maxilla
where it joined the premaxilla.
In occlusal aspect (Fig. 1C), tooth sockets 1 and 3 house fully developed but unerupted replacement teeth, sockets 4 and 5 contain newly
erupting teeth, socket 6 has a fully erupted tooth, and socket 2 holds a
broken crown. The sockets themselves are subrectangular or reniform,
with a slight inward curving of the medial wall of the socket. These
sockets are wider anteriorly than posteriorly. In addition, they become
longer posteriorly: socket 1 is 150 mm, and socket 6 is 180 mm long. The
ventral halves of the tooth crowns are symmetric in lateral view; only
fully erupted tooth number 6 is slightly recurved posteriorly. Denticles,
spaced at 2.3–3.0 per millimeter, adorn the entire anterior and posterior
carinae of each tooth, fading only at the very tip.
236
NOTES
FIGURE 1.
237
Specimen UCPC 10, stereoscopic pairs. A, lateral view. B, medial view. C, occlusal view. D, anterior view. Scale bar equals 5 cm.
DISCUSSION
Comparisons
UCPC 10 can be readily diagnosed as a theropod maxilla based on its
pneumatized antorbital rim, and its teeth, which are serrated, labiolingually compressed, and slightly recurved. UCPC 10 exhibits several
characters that suggest it as a primitive member of the Abelisauridae, a
basal group of Cretaceous theropods with a Gondwanan distribution.
These consist of a dorsoventrally deep maxillary body, a near vertical
articular edge for the premaxilla, pneumaticity limited to an uninflated
promaxillary fossa, fully fused interdental plates, rather small teeth with
little or no recurvature, subrectangular alveoli in ventral view, and pitted
sculpturing of the lateral surface (Bonaparte and Novas, 1985; Holtz,
1998; Sampson et al., 1998; Lamanna et al., 2002; Sereno et al., 2004). In
these respects, UCPC 10 is similar to the known abelisaurids Rugops
primus, UNPSJB-PV247 (an abelisaurid maxilla from Argentina), Abelisaurus comahuensis, Rajasaurus narmadensis, Majungatholus atopus,
and Carnotaurus sastrei. Abelisaurid skull bones from the latest Cretaceous are thicker and more ornamented in the more derived taxa, such
as Majungatholus (FMNH PR2100) and Carnotaurus (FMNH PR1950).
The lighter build and unornamented medial aspects of UCPC 10 suggest
it to be more primitive.
The Moroccan maxilla is particularly comparable to that of the Nigerien Rugops primus in tooth morphology, alveolar shape, rugose lateral texture, the straight border with the premaxilla, and the position of
the palatal shelf. The two maxillae differ in that the Moroccan specimen
shows more pitting on the lateral surface, while the Rugops maxilla has
more pronounced lateral channels and grooves. Also, the lower medial
surface of the Moroccan maxilla shows a series of convexities and is only
lightly striated; the medial surface of the Nigerien maxilla is flatter and
more heavily striated. In addition, the lamina medialis is slightly taller
than the lamina lateralis in Rugops, whereas these walls are approximately the same height in the Moroccan maxilla. Because many currently
known abelisaurids exhibit subtle differences in maxillary ornamentation
(e.g., Majungatholus atopus, Carnotaurus sastrei, and Rajasaurus narmadensis), these differences may indicate that the two maxillae represent
distinct species, as is the case with Cenomanian carcharodontosaurid
material from Morocco and Niger (Sereno, pers. comm.).
Like Rugops (Sereno et al., 2004), UCPC 10 is close in form to Argentine maxilla UNPSJB-PV247, especially in its degree of antorbital
pneumatization, and its subvertical premaxillary articulation, palatal
shelf, and rugose lateral sculpturing. UCPC 10 differs primarily in its
slightly smaller size and smooth, undulating medial surface.
The Moroccan maxilla shares some similarities with carcharodonto-
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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 25, NO. 1, 2005
FIGURE 2. Line drawing of UCPC 10. A, lateral view. B, medial view. Abbreviations: a1, alveolus 1; aofe, antorbital fenestra; ap, anteromedial
process; ar, ascending ramus (broken); df, dental foramina; f, flange (overlaps premaxilla); ip, interalveolar pits; ls, lateral sculpturing; mc, medial
convexities; ms, medial shelf (broken); mst, medial striations; pfo, promaxillary fossa; pr, posterior ramus (broken); ut, unerupted teeth. Scale bar
equals 5 cm.
saurids, such as the rugose lateral surface and steep maxillary - premaxillary suture. Also, the degree of curvature of the medial wall in UCPC
10 is similar to that present in Carcharodontosaurus saharicus. However,
in carcharodontosaurids, the maxillae are many times larger than that of
UCPC 10, the anterior surface of the maxilla (maxillary - premaxillary
suture as well as ascending process) is inclined to a greater degree, the
lamina medialis is broadly exposed in lateral view, and the antorbital
fossa is well developed. Also, the external rugosities on carcharodontosaurid maxillae are not as dense as those on UCPC 10, and carcharodontosaurid teeth are characterized by broader crowns with marginal
wrinkles.
Biogeographic Implications—One hypothesis of Gondwanan dinosaur biogeography argues that Africa had completely separated from the
rest of Gondwana prior to the abelisauroid radiation, and that abelisauroids spread from South America to India and Madagascar via circumferential land-bridge connections (Sampson et al., 1998, 2000; Krause et
al., 1999; and Carrano et al., 2002). This hypothesis was previously supported by the late (Campanian-Maastrichtian) temporal range of abelisauroids (except the putative abelisaurid Genusaurus sisteronis from the
Albian of France, which is considered indefinite by Carrano et al., 2002,
and is ignored by other proponents of this hypothesis) and by the lack of
abelisauroids in Africa.
UCPC 10, in combination with recent fossil discoveries of similar age
in Africa and South America, establishes the presence of abelisauroids
on Africa by the Cenomanian, weakening this hypothesis in favor of one
that postulates sustained faunal contact between Africa and South
America into the Late Cretaceous (Sereno et al., 2004). A disputed
Moroccan abelisaurid dentary fragment (Russell, 1996) and a Patagonian
abelisaurid maxilla (Lamanna et al., 2002) from the Cenomanian—
Turonian (concurrent with geologic separation of Africa from Gondwana) cast doubt that Africa had wandered into isolation before the
abelisauroid radiation. Discovery of a noasaurid and an abelisaurid from
the Aptian-Albian of Niger (Sereno et al., 2004), and the presence of
UCPC 10 in Cenomanian Morocco confirm an earlier abelisauroid radiation that included the African continent. The similarity of UCPC 10 to
Argentine maxilla UNPSJB-PV247 and the maxilla of Rugops primus
suggests very recent common ancestry of these three taxa, suggesting the
possibility of faunal interchange between Africa and South America as
late as the Aptian-Cenomanian.
Diversity—The Kem Kem beds have produced a disproportionately
large number of large carnivores, whereas herbivores of any size remain
extremely rare (Russell, 1996; Sereno et al., 1996). The confirmation of
an abelisaurid in Morocco brings the total number of large carnivores to
four, also including Spinosaurus maroccanus, Carcharodontosaurus saharicus, and Deltadromeus agilis (I consider Sigilmassasaurus brevicollis
material to be referable to Spinosaurus maroccanus).
Russell (1996) attributes the high abundance of theropods to deltaic
piscivory. This hypothesis assumes that there was minimal transport of
remains from upstream environments, which is difficult to maintain for a
deltaic facies. Also, only one group of Kem Kem theropods, the spinosaurids, is adapted to catch and eat fish (reviewed in Charig and Milner,
1997; Sereno et al., 1998). While spinosaurid fossils are common in the
Kem Kem, the confirmed presence of an abelisaurid adds another large
predator to the fauna, making it even more enigmatic.
Acknowledgments—I wish to thank P. Sereno for invaluable guidance
in all stages of development of this project and for discussing and reviewing many drafts. M. Coates and N. Shubin reviewed early versions of
the manuscript, providing many insightful comments. Also, Kenneth
Carpenter, Rodolfo Coria, and Eric Buffetaut reviewed my submission,
helping me to tighten and clarify it. I am grateful to J. Conrad for help
with specimen comparison, W. Simpson for access to casts, R. Masek for
fossil preparation, and C. Abraczinskas for help with line drawings and
photographs.
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Submitted 3 February 2004; accepted 28 April 2004.